1. Field of the Invention
The present invention relates to a laser velocimeter for near surface measurements, and more particularly, to a miniature dual-beam type laser velocimeter for measuring near-wall three-dimensional turbulence in a wind tunnel or similar flow environment.
2. Description of the Related Art
Computational fluid dynamics has reached a level where solutions of the Reynolds-averaged, Navier-Stokes equations for complex three-dimensional flows are practical. In many applications for which these equations are solved of primary interest is the effect of the fluid flow on a solid body, e.g., an aircraft or aircraft component. In these cases, the modeling of the turbulent Reynolds stresses in the near-wall region becomes a critical element in achieving accurate predictions of skin friction and heat transfer. For strong inviscid/viscous interactions, near-wall modeling is also important in predicting the mean pressure field. Up until now, the understanding of how these Reynolds stresses behave in the near-wall region under complex flow conditions and how they should be modeled has been poor. This lack of understanding is due in part to the paucity of accurate near-wall turbulence data. Especially lacking are near-wall data for flows that are highly three-dimensional and/or in some stage of separation.
For three-dimensional (3-D) boundary layer flows, -u'v' and -w'v' are the most relevant Reynolds stresses. Most three-dimensional boundary layer studies have relied on hot wire anemometry to measure these quantities. The technique requires a fine wire, for example, 5 .mu.m in diameter, mounted on two prongs and inserted into a flow. A current is passed through the wire. The faster the air flow, the more the wire cools. By measuring the cooling effect of the wire with respect to time, the velocity of the flow can be determined. The major shortcomings of hot-wire anemometry are its relatively poor spatial resolution near surfaces when the fluctuating velocity component normal to the surface v' must be measured and its inability to provide accurate measurements when turbulence levels are high. Because of its poor spatial resolution, measurements of -u'v' and -w'v' are generally limited to approximately 2 mm from a solid surface.
Laser velocimetry and, in particular, a dual-beam or fringe approach, offers the potential for expanding the near-wall turbulent flow database In the dual-beam approach, two mutually coherent laser beams are brought to a common focus in the flow. Local near-instantaneous fluid velocities are then determined by measuring the Doppler shift difference of laser light scattered by micron-size particles passing through the overlap region of the focused laser beams. Multiple velocity component, dual-beam laser velocimeters typically are multi-color systems. A two-velocity component system, for example, would consist of two pair of laser beams at two different wavelengths. Although dual-beam laser velocimetry is a very powerful technique, solid surfaces and the measurement of the crossflow velocity component w present problems. In addition, unlike hot-wire anemometry, as the test facility becomes larger, near surface measurements become more difficult with laser Doppler velocimetry since the measurement point becomes further removed from the optical components.
Solid surfaces present a problem for laser velocimetry because they can produce a large amount of diffusely reflected laser light. The diffusively reflected laser light that reaches the photodetector introduces noise into the signal which can cause measurement errors. Added noise can become so large that it overwhelms the low level signal bursts produced by the micron-size particles immersed in the flow. In which case, meaningful measurements become virtually impossible.
For boundary layer measurements, it is advantageous that the incident laser beams approach the wall of the object with a grazing incidence. Under these conditions, better spatial resolution normal to the wall is achieved and the amount of diffusively reflected light at the photodetector is reduced. However, the accurate measurement of the crossflow velocity component w is difficult with the laser beams at such a grazing incidence.
The velocity component sensed with a dual-beam laser velocimeter lies in the plane of the two incident beams and perpendicular to the bisector of the angle formed by these two beams. This bisector will be referred to as the optical axis. Congruent with the optical axis is the major axis of the ellipsoidal sensing volume located where the two laser beams intersect.
The u and v velocity components can be measured directly using overlapping sensing volumes having axes which lie parallel to the boundary-layer surface and perpendicular to the freestream flow direction. However, to measure the w component directly with the sensing volume axis parallel to the boundary-layer surface, the optical axis must be aligned with the freestream flow. Limitations in optical access usually preclude such an arrangement. Instead, the optical axis or sensing volume is inclined at an angle .theta. relative to the freestream flow. The resultant measured velocity component is equal to ucos.theta.+wsin.theta.. The larger .theta., the greater the sensitivity to the w component. In many facilities, however, limited optical access precludes making .theta. very large. In addition, when .theta. is large, the overlap region between the sensing volume for w and the sensing volumes for u and v becomes small, resulting in "virtual particle" measurement errors. Virtual particle measurement errors occur when signals from two different particles outside the overlap region are misinterpreted as coming from a single particle in the overlap region. These measurement errors can be quite large.
That is, a first problem is that if the dual-beam or fringe velocimeter system relies on a grazing incidence of the laser beams where the laser beams enter the flow from the side, sufficient sensitivity to the w component can be difficult to achieve. Direct measurements of the u and v components can be easily made but only partial sensitivity to the w component is possible because the velocity component sensed using dual-beam laser velocimetry is perpendicular to the optical axis. Usually, only small sensitivities to the w component are possible because of limited optical access. The smaller the sensitivity the greater the uncertainty in the w component measurement.
A second problem in the velocimeter system arises when the overlap region of individual sensing volumes of a multi-velocity component laser velocimeter is small in comparison to the individual sensing volumes. In this case, signals from particles outside the overlap region can cause "virtual particle" errors if the data reduction assumes coincident multi-channel measurements from the same particle. The virtual particle problem usually degrades the accuracy of the turbulent Reynolds stresses involving w.
Finally, there is the problem of deteriorating performance with an increase in scale. One way to obtain better resolution of the near-wall region is to generate a larger scale flow. In laser Doppler velocimetry, near-wall measurement capabilities degrade as the size of the test facility becomes larger because of practical limitations. One practical limitation is that the scale of the flow usually does not increase in direct proportion to the size of the facility. Another limitation is that as the optical components become larger they become poorer in quality. In addition, diffraction-limited performance and light collection at large solid angles become very expensive and difficult to achieve.